A cross-coupling reaction of a Grignard compound with an alkyl halide is employed for various reactions for generating a carbon-carbon bond.
For this reaction to generate a carbon-carbon bond, a transition metal compound such as palladium, nickel and copper is generally used as a catalyst. In addition, there are many reports showing that reaction yield for across-coupling reaction is improved by combination of these transition metal compounds and various additives. Specifically, it is presently known that yield is improved by using phosphine with a nickel catalyst or a palladium catalyst (Non-patent Document 1), or by using a copper catalyst with olefins such as 1,3-butadiene (Non-patent Document 2).
Further, a method of improving reaction yield by adding polar substances has also been reported. For example, it has been reported that yield for cross-coupling using dilithium tetrachloro cuprate catalyst can be improved when a polar solvent such as N-methylpyrrolidinone is added in an excess amount (i.e., 4 to 6 equivalents) relative to a Grignard compound (Non-patent Document 3), or solubility of a n-propylethynyl copper reagent in ether (solvent) and reaction yield are improved when two equivalents of hexamethyl phosphorous triamide is added for synthesizing an organolithium reagent from a n-propylethynyl copper reagent (Non-patent Document 4).
A branched fatty acid is widely used as additives for an industrial product, intermediate materials, and raw materials for cosmetics and perfumes. For example, isostearic acid having an excellent property such as emulsifying property is incorporated in various cosmetics and perfumes. Further, along chain anteiso fatty acid, which is a branched fatty acid present on the surface of human hair, is a useful fatty acid as being capable of protecting hair and providing hair with smooth feeling.
However, the long chain anteiso fatty acid having such important properties is mainly extracted from wool and used. Since isolation of a specific long chain anteiso fatty acid is a very difficult process, its use is limited to a mixture containing them.
As for a method for the selective production of a branched fatty acid, the cross-coupling reaction as described above, and a method based on reaction for generating a carbon-carbon bond as exemplified below have been reported. However, these methods are not all industrially convenient and they cannot be performed at low cost.
(I) A method including reacting dihaloalkane with haloalkyl magnesium in the presence of a copper catalyst such as dilithium tetrachloro cuprate and subsequently carboxylating the terminal group (Patent Document 1), and a method including reacting dicarboxylic monoester with an organic cadmium compound and reducing the resulting keto acid (Non-patent Document 5).
These methods are problematic in that, as including a step of converting selectively one of two identical functional groups, yield is low. Further, although another method in which one of the two identical functional groups is protected with a protecting group and reacted has been reported (Non-patent Document 6), it is also problematic in that the number of the required reaction processes is big and operation is roundabout.
(II) A reaction for generating a carbon-carbon bond between two kinds of a Grignard compound and halocarboxylic acid (Non-patent Document 7).
Because two kinds of a Grignard compound are used, this method is problematic in that general applicability of a substrate is low, and it is unfavorable in terms of production cost. In addition, while it is believed that 11-bromoundecanoic acid as halocarboxylic acid used is synthesized from 10-undecenoic acid, descriptions or method of obtaining halocarboxylic acids having eleven or more carbon atoms is not included and production of a long chain branched fatty acid is not established.
(III) A method based on Wittig reaction
i) A method in which an aldehyde and a phosphonium salt obtained from diol are subjected to a Wittig reaction followed by reduction and carboxylation of the terminal group (Non-patent Document 8).
ii) A method in which an aldehyde and a branched alkyl phosphonium salt that is represented by the following formula (Y) are subjected to a Wittig reaction followed by reduction and hydrolysis (Patent Document 2).

(In the formula, R′1 represents a methyl group or an ethyl group, R′2 represents a saturated or unsaturated hydrocarbon group, R′3 represents an alkyl group or an alkenyl group having 1 to 6 carbon atoms and m represents an integer of 4 to 16.)
iii) A method in which ω-phosphonium fatty acid ester represented by the following formula (Z) and a branched aldehyde are subjected to a Wittig reaction in the presence of a base such as sodium methylate followed by reduction and hydrolysis in the presence of a basic catalyst (Patent Document 3).

(In the formula, R″1 represents an alkyl group or an alkenyl group having 1 to 6 carbon atoms, R″2 represents a saturated or unsaturated hydrocarbon group, R″3 and R″4 respectively represent a methyl group or an ethyl group, X″ represents a halogen atom, k represents a number of 5 to 16 and j represents a number of 0 or 1.)
However, these methods including the Wittig reaction are problematic in that reaction steps are too many and a great amount of phosphine oxide is produced as byproduct.
(IV) A method in which a carboxylic ester is produced from a Grignard compound and a halocarboxylic ester in the presence of copper compound followed by hydrolysis to obtain a branched fatty acid (Patent Document 4).
According to this method, a side reaction between an ester and a Grignard compound occurs in addition to a desired cross-coupling reaction. As such, not only yield is lowered but also separation of a byproduct remains as a problem.
Meanwhile, with respect to a ω-halo long chain carboxylic acid which is useful as a starting material for producing a branched fatty acid, etc., a method of producing 15-bromopentadecanoic acid by Kolbe electrolysis of azelaic acid, forming half salts of Ag, and bromination (Non-patent Document 9) and a method of producing 15-bromopentadecanoic acid by reacting 15-methoxypentadecanoic acid with boron tribromide (Non-patent Document 10) and the like have been previously reported. However, these methods all require many steps, and therefore are not a method for convenient synthesis.
In addition, a method in which 15-bromopentadecanoic acid can be obtained in high yield from 15-pentadecanolide (trade name Pentalide) by using conc. sulfuric acid-hydrogen bromide acid has been reported recently (Non-patent Document 11). However, according to this method, there are some problems such that the reaction system becomes blackened, or insoluble matters are produced and cannot be separated easily from an aqueous layer so that refluxing for a long period of time like 3.5 days is required. As such, it is not applicable for actual synthesis.    [Non-patent Document 1] Bull. Chem. Soc. Jpn., 1976, 49, 1958-1969    [Non-patent Document 2] J. Am. Chem. Soc., 2003, 125, 5646-5647    [Non-patent Document 3] Tetrahedron, 2000, 56, 2733-2737    [Non-patent Document 4] J. Am. Chem. Soc., 1972, 94, 7210-7211    [Non-patent Document 5] Biochemistry, 1987, 26, 4636-4044    [Non-patent Document 6] Eur. J. Lipid Sci. Technol. 2003, 105, 627-632, Biosci. Biotechnol. Biochem. 2001, 65(2), 463-465    [Non-patent Document 7] Tetrahedron Lett., 1976, 51, 4697-4700    [Non-patent Document 8] J. Org. Chem., 1986, 51, 2751-2756    [Non-patent Document 9] Synthetic perfumes, The Chemical Daily Co., Ltd.    [Non-patent Document 10] Bull. Chem. Soc. Jap., 54(3), 945, 1981    [Non-patent Document 11] Aust. J. Chem. 51, 581-586, 1998    [Patent Document 1] JP-A-S60-161946    [Patent Document 2] JP-A-H05-025108    [Patent Document 3] JP-A-H06-128193    [Patent Document 4] WO 2006/083030